Platinum crucible



Jan. 3, 1939. J. s. sTRElcHER PLATFNUM CRUCIBLE Filed Nov. 17,

1934 Sheets-Sheet 1 F/G. .l

GPA MS CPUC/BLE WEIGHT 1N V EN TOR. annjurez'celr BY Q5 W L A TTRNEYS.

Jan. 3, 1939. J. s. sTRl-:ICHER 2,142,660 Y,

vPLATINUM CRUCIBLE l Filed Nov. -17\, 1934 2 sheets-sheet 2 CPUC/BLEWEIGHT GPA/VIS )IC/G.

. Byww ATTORNEYS.

Patented Jan. 3, 1939 v PLATINUM cnuolns Jol'mm s. stretcher, Newark, N.1.. signor to' The American Platinum Works, Newark, N. J., a corporationo! New Jersey Application November l'l, 1934, Serial No. '153,418

^ One oi the most important tools of the analytical chemist is theplatinum crucible. Since platinum belongs to the relatively rare metalsit is a high priced metal. This iact alwayshas been it and still istoday the reason'ior making the platinum crucibles with the leastpossible amount of platinum; the crucibles are made from such thinplatinumsheets as to give the crucibles just the strength necessary fortheir use in the chemical laboratory. 'I'his rule has been applied tothe manufacture oi the platinum crucibles for generations; today a tacitagreement exists between the chemists and the manufacturers o! theplatinum crucibles which restricts the manufacture of the standardplatinum crucible to this rule of thumb: Platinum crucibles togetherwith their covers shall weigh approximately as many grams as theyholdcubic centimeters. Thus a platinum crucible with. aA capacity of 25cubic centimeters weighs 25 grams (with a tolerance of .5 m) includingthe cover. The weight of the platinum cover of this crucible is about W2grams; the weight of the crucible proper is, therefore, only about 20grams. (Generally the .platinum covers weigh from Mlth to Vath of theaggregate weight of the crucible andv cover.)

This rule of thumb determines and limits absolutely the thickness of thecrucible bottom and side wall. The crucibles are made from platinumsheets; these Sheets are cut into circular discs or blanks and are spunover spinning forms made oi' steel or wood; during the spinningoperation the circular blanks are pressed with a follower against thesteel form; the follower has a iace which nearly covers the bottom ofthe crucibleshaped steel form; in this manner a crucible bottom isobtained which always has the thickness of the original sheet; thecrucible wall becomes slightly and' gradually thinner from the bottomtowards the rim ci?v the crucible. Instead of spinning, other processessuch as for instance pressing or deep drawing, are also used for formingthe circular blanks into 'crucibles of the shape just referred to.Produced according to this rule 45' oi thumb platinum crucibles of themostwidely used standard sizes have the following dimensions:

lWhen for instance .014" platinum sheet is spun into a 25 cubiccentimeter crucible, the thickness of the sheet is reduced, fromthebottom towards the rim, from .014" to .010 vor .009"; sheets of adierent gauge are reduced in thickness to an analogous extent, generallyin about the proportion indicated.

Pure platinum as well as the well-known platinum alloys (crucibleplatinum with .3% indium or rhodium, or platinum-rhodium with 3.5 to 4%rhodium) are treated alike when shaped into crucibles; theplatinum-rhodium crucibles sometimes are made from even thinner sheetsthan the pure platinum crucibles; these alloys vare slightly harder thanthe crucible platinum.

The'metal of all the platinum crucibles has an extremely finecrystal-grain. This fine crystalgrain ,is attained by the mostpainstaking hammering of the metal after the spinning process. The metalot the commercial platinum crucibles has a grain varying in size from.25 to .66 millimeter, or about 18 to 2 grains per square millimeter(measured at the bottom of the crucibles) Chemists as well asmanufacturers consider those crucibles as of the best qualities whichhave the smallest crystal grain.

I'here are, according to my experience and research, three main causesof the destruction oi platinum crucibles during their use in chemicallaboratories:

1. When the crucibles are used for holding molten masses of alkalicarbonate or potassium bisulfate, the thin, hot and therefore extremelysoft and pliable crucible metal sags under the weight of the melt. Thecrucibles are considerably distended along the bottom section and asfarup as the melt is lling the crucible; the crys- .tals of the cruciblewall are stretched; the intercrystalline spaces are enlarged. When themelts cool slowly within the crucibles or when they are quenched, themelts expand slightly and cause a secondary stretching of the metal.Both of these actions change the shape of the crucibles very rapidly.Wooden or other forms are orten used to iron out these bulges; thisprocedure l very often causes. cracks within the metal. The alkalicarbonate melts also 'contribute towards the destruction of the platinumcrucibles chemically. These melts corrode platinum slightly; but thiscorrosion takes place especially along the edge 2. Platinum cruciblesare most frequently heated with gas burners (Meker burners, etc.); thecrucibles are heated in such a way that they are completely enveloped bythe blue coneof the gas llame. This blue conev of the gas flame containshydrogen (the amount of hydrogen in this blue cone is regulated by thelaw of the water gas reaction). Wherever this blue cone of any gas ilametouches the platinum crucible, hydrogen dliuses through the platinum andcausesreducing processes within the melts; metal oxides are reduced tothe metals; these metals alloy withthe platinum very readily and destroythe crucibles rapidly.

3. Every platinum crucible is handed to the chemist in the hard (coldworked) and highly polished state. Heating the crucibles for the iirsttime (in the gas burner or in an electrically heated muille; thetemperature attained is seldom higher than llElllo C.) causes instantrecrystallization oi the metal. The distorted grain structure, attainedby forming and hammering the crucible, is replaced by new, small andnormal grains (an cquiaxed grain, Whose diameter is approximately thesame in all directions). The strain hardening eilect is completely lostwith thisv change of structure; the metal becomes very soft and ductile.Dining the use of these crucibles it very often happens that, atrestricted areas or places, an exaggerated grain growth takes place andan abnormally large grain is produced. Such exaggerated grain growth is'the cause of many cracks in platinum crucibles.

When, in the chemicai laboratory, platinum ware is destroyed, thesethree causes of destruction are not always equally active; theconditions which cause such .i allures vary with each laboratory; theyare very seldom recognized and identified individually. Thereforefailures of platinum. ware are very seldom traced to their true source.l.In chemical literature they are, mostly recorded as interestingphenomena, essentially connected with the nature of platinum metal andtherefore unavoidable. But another notion obscures the underlying facts.The platinum cruclble is made today, with only minor changes, accordingto a tradition nearly 75 years old. Neither the process of making thecruclble, nor the hud of-crucible produced by this process is consideredto have any bearing upon the failures of the crucibles when in use. Theplatinum crucible of yesterday and of today is really taken from thehands of the manufacturer like a ,constant of nature and in case anyfaults are found, the. blame is put primarily upon the chemical qualityor purity of platinum. But since platinum is generally refined to a muchhigher degree than any other commercial metal ever was rened, thechemical quality of platinum is least to blame for any failures. v

Through an extensive research I have found that when the platinumCrucible is varied in its physical structure the metal shows Widelyvarying behavior under the conditions under which platinum crucibles aregenerally used. In producing such variations Vin the structure of themetal it was necessary to stop giving primary importance to the purelycommercial viewpoint of the price of the metal; it was necessaryI tofollow a line of development basedv solely upon the properties ofplatinum metal itself.V These tests have enabled me to devise my presentinvention, showing a. way of making a platinum crucible which combineswith a least amount of platinum the highest resistance to corrosion,which reduces the dif fusion of the hydrogen of theilame through thecrucible walls to a minimum and which overcomes the possibility of thechanges in the shapes of the crucibles during the fusion of alkalicarbonate.

My researches showed that platinum can be best studied in its chemicaland physical behavior lwhen shaped into a large number of crucibles 4ofthe same capacity but with different bottom and wall thickness Aand whenin such crucibles alkali carbonate melts are made under varying butalways exactly determinable conditions. These tests and their resultsare of special advantage, as they correspond absolutely to the practicalcase in the chemical laboratory. In addition thereto, special tests havemownfthat alkali carbonate melts corrode platinum only insofar as theyare dissociated to alkali hydroxide or alkali oxide and when oxygen, airor such gases as produce oxygen through dissociation (for instancecarbon dioxide) are present. Soda ash and potash start to dissociateinto the oxides at 300 C.; the higher the temperature, the more alkalioxide is produced within the melts (potassium carbonate is decomposed toa higher degree than sodium carbonate at the same temperatures), thegreater is the corrosion, therefore, of the plati num crucibles by thealkali carbonate melts. The alkali carbonate melts act upon theplatinum. under oxidizing conditions, not only in a purely chemical Way;as tests have shown, the process of corrosion is also activated by theplatinum itself and especially the platinum oxides which are formedduring the initial corrosion and which are oating within the melt.Platinum is never attacked or corroded by alkali carbonate melts in apurely reducing atmosphere; on the contrary, in such systems hydrogenreduces to the alkali metals, thetraces of alkali oxide formed above 800C.; these metals are then deposited upon the inner surface of theplatinum crucibles; these alkali metal deposits alloy with the platinum.Such melts, made under strictly reducing conditions, are alwaysbrilliantly White. l the alkali carbonate melts are made in a Inutilewhich is electrically heated and Where there is always an excess of air,fusion proceeds under purely oxidizing conditions. Tests have shown thatthis is the only way in which it is'possible to obtain exact corrosiondata for platinum at diierent temperatures. 'I'he melts resulting fromsuch fusions are slightly colored (brown) by the platinum oxide whichis'formed by the corrosion of platinum; since the corrosion rateincreases with the length of time and with higher temperatures thecolorof the melts is also intensied when the fusionsare carried on for alonger time. When a gas burner is used for heating the platinumcrucibles during these fusions, corrosion of the platinum also takesplace, but the rate of corrosion is quite different from the above case;the melts `corrode the platinum in a minor degree. Special tests haveshown that'the corrosion depends upon the rate with which )the hydrogenwithin the flame diluses through the platinum into the melt and upon thedegree with which the ame enveloping the crucible -blocks the air fromentering the melt freely. These are the two factors which cause thereduced corrosion of the platinum when.

the crucibles are heated with a gas flame.

Applyingthe testing method. explained above, to platinum crucibles withdiilerent bottom and wall thiclmess, there were ascertained a series ofnew and very important facts Awhich had never Vbeen taken into accountin the manufacture of corosion decreases with the increase in thethickne s of the platinum sheet and with the increase in )the size ofthe crystal grain.

f2. The corrosion is leastin crucibles made from platinum sheets ofa'thickness of .020" and over.

3. When the platinum crucibles are heated with a gas burner (Mekerburner, etc.) the corrosion of platinum decreases wit thickness of thecrucible. 'I'he fusion proceeds under reducing conditions in cruciblesmade from.

sheets .018" thick and thinner. The reducing action within suchthin-walled crucibles is'caused by the diusion of the hydrogen ofthellame through the platinum into the melt.

4. Platinum sheets with a thickness of .020

and more act practically as an impervious shield against the hydrogen ofthe flame; within platinum crucibles made from sheets .020" @andthicker, the fusion of alkali carbonate proceeds under oxidizingconditions similar to those obtaining with an electrically heatedmuiile.

5. During thefusion of alkali carbonate, platinum crucibles made fromsheets .020" thick and more will sag'but slightly or not at all lunderthe weight of the melts. Thin walled' platinum crucibles (thinner than.020") sag very quickly and easily under the weight of the melts.

'I'hese tests and their results show that the thin walled crucible whichis made from sheets thinner than .020" (this includes all the commercialstandard platinum crucibles, as they are made from sheets varying inthickness from .010 ,-to .0175", according to the volume by weightrule), with the extremely iine crystal structure y is in a physicalstate where it is most susceptible to destruction in the regular routinework of the chemical laboratory. These thin walled crucibles show theleast resistance to the difusion of the hydrogen of the flame. extremelyne crystal grain, they develop the highest catalytic activity whentheyare heated under oxidizing conditions; therefore, they show thehighest corrosion rate with the alakali carbonate melts. 'The thin andfinely' crystalline platinum sheets are extremely soit when in the hotstate; thereforathin walled crucibles will sag and change their shapequickly under the weight of the melts. l

The tests mentioned above indicate that the lack of stability of thestandard platinum crucibles is overcome completely when the crucibles(of whatever capacity) are made from sheets of a thickness of at least.020".or more. My invention vis based on these tests, and enables me toproduce platinum crucibles `far superior to the standard crucibles nowemployed, as to durability, permanence of shape, and resistance todeleterious influences such as corrosion by the melt contained in thecrucible or impairment by the action of the hydrogen of a llame used forheating the crucible.

' Thel superior results obtained by my invention appear very clearlyfrom the data shown inthe accompanying drawings, in which Figs. 1 and 2are diagrams illustrating the corrosion conditions observed whencrucibles ofv a predetermined capacity (here assumedv as 25 cubiccentimeters) are made from platinum sheets of different thicknesses andof different character and exposed to a decrease in the wallv As theyhave an v the action of two diilerent melts; Fig. 1 represents theresults obtained when an electrically heated mume is employed, whileFig. 2 illustrates those observed when the crucibles are heated with aMeker gas-burner; Fig. 3 is a vertical section oi a crucible madeaccording to my invention from,V

finely-crystalline platinum; and Fig. 4 is a simi` lar view of aplatinum crucible the metal of which is of the so-called single crystalcharacter.

According to extensive tests, the best resultsI are obtained with myinvention by using platinum sheets .025" thick. For the same volume orcapacity, the new crucibles embodying my invention are heavier thanstandard crucibles such as now employed, as will be evident from thefollowing table giving the approximate weight of platinum crucibles(crucible alone, without the cover) made from platinum sheet .025 thick:

New crucible, Standard Incxeo Volum .c25/'wan crucibles in weight Cubiccmimeten Grams Grams Gram l0 22 8 14 l5 28 12 i6 zo a4 1e 1s 25 40 20 Z)30 46 24 22 40 54. 32 22 50 62 40 22 so 7o so eo taken as .025", theline A indicates that the the portion of the line A from the point A'upward (thicknessl .020" or over) indicates the range of thickness to beemployed according to my invention. As will be explained below, thepointl A vpoints of said line below A'.

maybe termed a critical point ofthe line A, in

that conditions are materially diierent for points of the line A aboveA' from the conditions for With an increase in weight from 14 to 22grams of platinum for the standard sizes of the chemical laboratorycrucible, the main and most powerful factors tending to destroyplatinumcrucibles are entirely eliminated and overcome, namely on the.one hand, the diffusion of hydrogen into the melts when gas burners areused and the subsequent destruction of the crucible through chemicalaction; and on the otherhand. the sagging of the crucibles and theirsubsequent destruction throughmechanical causes. With the new thick-Wallcrucibles of my invention the fusion of alkali carbonates proceeds underoxidizing conditions even when gas burners are used for heating f thecrucibles. The modern textbooks of quantitative analytical chemistryadvise the use of a small amount of niter together with the alkalicarbonates in order to i'nsure positively an to platinum and causes aplatinum sponge deposit upon the inside surface of the thin walledcrucibles. From such crucibles the cold melts can be separated only withdiiiiculty; applying mechanical ways toseparate the melts from suchcrucibles destroys the thin walled crucibles very quickly. The new thickwalled crucible is least corroded by the alkali carbonate melts; theinner surface of the crucibles remains smooth for a very long time; themelts are very evenly heatedv within such crucibles;A the cold melts aremost easily separated from these4 crucibles. 'Ihese improvements areattained with all the thick Wall crucibles made from C. P. platinum,Crucible platinum? as well as from the platinum-rhodium alloy containingup to 4% Rh.

As an example, I may produce platinum crucibles embodying my invention,in the 25 cubic centimeter size, as follows: Crucible platinum is rolledto a thickness of 0.120" and annealed at 800 C. for `three hours. Thisplatinum sheet is then rolled down to sheets of different thicknesses,each sheet being of uniform thickness, and the several sheets varying inthickness from .100" to .004". From-each sheet I cut two circular piecesor blanks; about 66 millimeters in diameter, and anneal these for 15hours at 800 C.

. Thereupon all these blanks are spun to i'orm the -ist (2.l (theresults being-shown'in Fig. 2).

shells or bodies of crucibles of 25 cubicl centimeter capacity, and suchshells or bodies are iinished by hammering or other suitable operation.One crucible of each pair is recrystallized by heating to 1000 C. andthen kept in the resulting finely crystalline state. The other crucibleof each pair is transformedinto a single-crystal crucible according to amethod described below. When I speak of the thickness of the cruciblewall, I mean that of the crucible bottom;l the side wall vis usually ofsomewhat smaller thickness, as has been stated above.

'Ihe crucibl of both types or series are subjected'to corr on tests asfollows: Each test is made for a period of one hour. In one series oftests, I produce within the-crucible (of 25 cubic centimeter capacity) amelt of 20 grams of soda (NanCOs), in another series of tests the meltconsists of 20 grams of a mixture of 4 parts of soda -ash (NaaCOa) and 5parts of potash (KzCOs). 'I'he fusion of the substance from which themeltv is 'formed may be accomplished in an electrically heated muiile at1000 C. (Fig. 1 illustrates the results of this treatment), or with theaidof a Meker blast gas burner at 1050 In the drawings, the lines B, C,D, E, B', C', and E' indicate the corrosion losses (due to the actionoi' the hot melt on the platinum) with diierent melts,

diierent heating agents, and diiIerent weights and'characters of theplatinum crucible.; For

.each of these lines, the abscissae indicate crucible weights in grams,and the ordinates,.c9rrosion losses in milligrams. Unes B, C, D, Erelate to fusion in an electrically-heated muiile, and lines B'. C',1'."A toheatixigV with the aid of ,a Meker blast gas burner. Lines lB,C, B', `C' relate to the corrosive action of a mixture oi soda andpotash of the composition mentioned above, while lines D, E, E relate tothe corrosive action of a soda melt. The light lines B, D, B' illustratecorrosion in the case oi' crucibles in the iinely crystalline state,while the heavy lines C, E, C', E' indicate the degree of corrosion inthe case of crucibles of the single-crystal type more fully describedbelow.

Fig. 1 shows clearly that with a crucible made from a platinum sheet.020" thick (point A' of line A, indicating a crucible weight of 30grams), the corrosion rate approaches its lowest and approximatelyconstant value, in the case of an' electrically heated muille. 'I'hecorrosion rate reaches its minimum when the sheet is .025" thick(crucible weight 40 grams). Thin-Walled crucibles have the highestcorrosion rates on account of the greater chemical activity of the smallcrystal grains. Under otherwise like conditions, the corrosion rate isgenerally smaller when the crucibles are of the single crystal type, ascompared with crucibles of the finely crystalline type.

Fig. 2 shows that when the crucibles are heated with a Meker burner, thecorrosion rate approaches its peak with the crucible made from aplatinum sheet .020" thick. With this particular mode of heating, theythin walled crucibles (less than .020" thickness) show the lowestcorromon rates, on account oi the high rate of dirfusion of the hydrogenof the ilame through the crucible walls. Comparison of the lines B andC' shows that in this case also the single-crystal l cruciblesarecorroded less than the iinely crystalline crucibles.

The third main cause for the destruction of the standard platinumcrucible is exaggerated grain growth oi.' the metal, caused by secondaryrecrystallization of the pure metal or the crucible platinum (chemistscall it mostly recrystallization). This harmful, locally restrictedexaggerated grain growth of platinumis caused, as tests have shown. whenthe recrystallized metal is in some places slightly bent or hammered andafter such locally restricted mechanical treatment again heated to 1000C. and above. Such mechanical treatment causes critical and localstrainsor strain gradients within such metal which is alreadyrecrystallized and which has been transformed through suchrecrystallization into an extremelyiine grain; these strains aresuddenly released when the crucibles are heated again to 1000 C. andabove; they cause the small grains to coalesce locally to extremelylarge grains. Platinum-.crucibles with nests of such large grainsdevelop cracks within such nests or at the boundaries where large andsmall grains meet. The changes in shape, caused by the alkali carbonatemelts when contained in the standard, thin-walled and finelycrystallin'e crucibles always inducethe chemist, after each fusion, toapply' to the crucible ythe above characterized mechanical treatment,fto reshape the'same; but such "reshaping operations result, sooner orlater, in exaggerated grain growth and ultimately in the destruction .orthe crucible. As has been stated above, platinum crucibles with a bottomthickness of .020 and more, especially those from .025"` upward, do notchange their shape under the weight oi.' the melts; therefore, theynever needfreshaping after the fusions.

By the use of my improved thick-walled crucibles, v

therefore, the third cause of destruction of the arranco' l't'is,however, possible to overcome this third cause for the destruction oftheplatinum crucibles entirely, by transforming the ne crystalline beimproved to such a degree that the crystals in sheets and wires willgrow to sizes up to 1- centimeter. and even more in length and width.When this method is applied to the finely crystalline platinumcrucibles, the metal of the crucibles is transformed in itsA entiretyinto an aggregate of such single crystals which have a thickness equalto that of the orucible wall and a' length and width many times thethickness of the cruclble wall; the whole cruclble consists ofI a fewtightly packed, large single crystals. These large crystalsfare all ofthe lsame order i of magnitude, that is to say, the cruclble contains nosmall crystals. The term uniform-large crystals as used in theappended'claims, is to be understood as stating that the cruclbleconsists exclusively of such large crystals. These single crystalcrucibles have features and charactera istics analogous to those of theWell-known single d 2. This hard-worked cruclble is quickly heatedthroughout to 1200" CA. and above, preferably to l600 C. and above. Theorucible is kept at these temperatures from l/2 to 2 hours. The metalrecrystallizes instantly to a line and normal'grain. The metal is thenin the dead soft state.

3. This platinum cruclble (recrystallized completely and transformedinto the dead soft state) is now hammered lightly with a highly polishedsteel-hammer all over its surface. The reduction in thickness resultingfrom this hammering treatment sh preferably not exceed 10%;` I havefound t best results are-obtained when such reduction in thicknessamounts to about 3%. The best way to attain such a mechanical treatmentis to take care never to apply the hammer twice to the same spot.Through such mechanical treatment critical strain gradients are createdall over and within the dead soft metal.

e. This still finely crystalline, `but dead soft and very lightlyhammered orucible is quickly heated to l200 C. and more, preferably to1600 C. and more and is kept at these high tem- Special tests have shownthat with the same metal and the same wall thickness the sinslecrystalcrucibles are thebetter, the nner the crystal grain of the originalorucible is. According to lthese tests the ilnestgrain of the metal isobtained when the platinum sheetused for the shaping of -the orucibleshells is reduced im thickness at least 75%, when rolled to the specialsize (best results are obtained with a 'reduction of 85% and even more).This reduction in thickness is generally performed in two or more stageswith intermediate annealing, and these intermediate annealing operationsare each conducted .800 C.- and for a time of from 1 to 15 hours.

These working conditions should also be observed in the method ofproducing the orucible shells by the spinning process and to thenecessary intermediate and nal annealings before the shells arehammered. The method of producing the single crystal crucibles can beapplied to the commercial cruclble platinum (platinum .with .3% iridiumor rhodium), and especially to thev different kinds of the C. P.(chemically pure) platinum. The size of the single crystals increaseswith the purity of the metal. This method of producing single-crystalcrucibles isv not applicable, however, to an alloy containing platinumand more than 2% of rhodium. The method of producing single-crystalcrucibles can be applied to all the cruclble sizes, to the thin-walledas well as to the ,thick-walled crucibles. The best results are obtainedwith the thick-walled crucibles, which are made from sheets ,020" to.060" thick, especially those .025" thick. Tests have shown that thecritical strain gradients are most easily attained with thesethick-walled crucibles through the hammering process explained above.With a sheet .025" thick, single crystals with an average length andwidth of more than six times the wall thickness are easily attained whenorucible platinum is used; when C. P. platinum is used, "single crystalsmeasuring in length and width up to 20times the diameter of the cruclblewall (made from the .02.5" sheet) are obtained and even larger crystals.In any event, I prefer that both the length and the width of these"single crystals should be at least three times the thickness of thecruclble wall. With the thin walled crucibles the slightest strokes withthe lightest hammer cause reductions' and strains within the dead softmetal which exceed the critical range. When the sheets are thicker than.060" they are dimcult to work; very strong strokes have to be 1appliedto them; these strong strokes cause at the surface of the metal a highreduction whereas the core of the metal is hardly inuenced by themechanical treatment.

Platinum metal when transformed into the single crystal structurechanges its qualities considerably. The `finely crystalline cruclblealways has rather a dull tone in its color when in the highly polishedstate; the metal of the singlecrystal cruclble has an extremely highbrilliancy; th metal attains this state of high brilliancy with thefirst secondary recrystallization, when it is annealed at 1600 C. Thisdifference in color is most easily observed with artificial light.ySince the brilliancy of the single-crystal orucible is its original andnatural condition, it is never necessary to polish the surface of such acruclble. The standard platinum crucibles such as made prior to myinvention, are polished with iron oxide or chromium oxide; .smallamounts of these substances always stick to the surface of the metal:-

the single-crystal crucible has an absolutely clean surface. The naturalbrilliancy of the singlecrystal crucible is very stable; only the mostsevere corrosive action can destroy this brilliancy.

'The forces which cause recrystallization and exaggerated grain growthin the fine crystalline crucible are completely deadened within thesingle-crystal crucible; the single-crystal crucible is' thereforeabsolutely stable in this respect. Therefore repeated rehammering andsubsequent reannealng is never harmful to the single-crystal crucibles(as it is to the thin-walled, standard and also to the thick-walledfinely-crystalline platinum crucibles); such repeated rehammering andreannealing always improves the thick-walled single-crystal crucibles.Since the metal of such single-crystal crucibles is extremely ductile,slight changes in the shape of such crucibles are corrected veryeasily.- The metal of the single-crystal crucible is transformed intothe highest state of passivity; therefore such crucibles are chemicallyless corroded than the finely crystalline crucibles. At extremely hightemperatures (1200 to 1700 C.) the metal of the single-crystal cruciblesis slightly less volatile in atmospheres which contain oxygen, than thefinely crystalline crucibles.

Taking-into consideration all the facts mentioned before concerning theadvantages of the heavy-walled platinum crucibles (.020 to .060" wallthickness, especially those which'have a bottom thickness of .025") andthe changes resulting from the transformation of such iinely crystallinecrucibles into single-crystal crucibles, it is most evident that thesingle-crystal crucible with a heavy wall constitutes the most perfecttype of platinum crucible. In this type of crucible, with the use of theleast possible amount of platinum, the following advantages arecombined:

1. It is the most perfect kind of a single-crystal crucible. All theforces which could cause defects through recrystallization areabsolutely killed.

l2. Greatest mechanical stability; the crucibles do not sag under theinfluences of alkali carbon-- ate melts; there is never any necessityfor reinforcing the crucibles at the rim 'or for reshaping them.

d polishing powders. Natural, clean surface with high briilia'ncy.

The principles outlined above for making an .bottom of the crucible. V/

improved platininn crucible can be applied to any shape of the platinumcrucible used in the chemical laboratory, to the standard shape of thecommon platinum crucible (these crucibles have the shape of a truncatedcone. the smaller end of which is used as the bottom)to the manyvariations between this shape and the simple, cy lindrically shapedcrucible, as well as to the recently introduced shapes which are knownas Interchangeable and combination crucibles.V

Fig. 3 of the accompanying drawings illustrates a crucible made fromiinely crystalline platinum and embodying my invention, while Fig. 4shows a similar crucible made from single-crystal material.

The term "platinum as used in the appended claims is to be interpretedas including the vari` ous kinds of platinum which I have stated to besuitable for use in connection with my present invention, that is tosay, so-called crucible platinum, as well as C. P. (chemically pure)platinum and platinum-rhodium alloys of the character explained above.

Various modifications may be made without departing from the nature ofmy invention as defined in the appended claims.

I claim:

1. A platinum crucible made of platinum in the completely unstrainedcondition and having a wall composed of uniform large crystals at 30least a majority of which are of a thickness substantially equal to thatof the crucible wall and of a length and width at least three times thethickness of the crucible wall.

2. A platinum crucible made of platinum in the completely unrestrainedcondition and having a wall composed of uniform large crystals at leasta majority of which are oi'. a thickness substantially equal to that ofthe crucible wall,

such wall thickness being at least .020" at the 40 bottom. y 3. Aplatinum crucible made of platinum in the completely unrestrainedcondition and having a wall composed of uniform large crystals at leasta majority of which are of a thickness substantially equal to that ofthe crucible wall, such wall thickness being .025" at the bottom.

4. A platinum crucible made from C. P. platinum in the completelyunrestrained condition and having a wall composed of. uniform largecrystals at least a majority of which are of s.

thickness substantiallyequal to that of the crucible wa such wallthickness being .025" at the 5. A platinum crucible made of platinum thecompletely unrestrained condition and having a wall composed of uniformlarge crystals at least a majority of which are of a thicknesssubstantially equal to that of the crucible wall. such wall thicknessbeing within the range of from .060" at the bottom.

